Unbreak compilation on non-Linux systems (FreeBSD)

and put the void cast of netlink_warn under the same preprocessor
condition as its definition, #ifdef HAVE_LINUX_NETWORK.

src/dnsmasq.c

@@ -81,7 +81,9 @@ int main (int argc, char **argv)
   int tftp_prefix_missing = 0;
 #endif
 
+#ifdef HAVE_LINUX_NETWORK
   (void)netlink_warn;
+#endif
   
 #if defined(HAVE_IDN) || defined(HAVE_LIBIDN2) || defined(LOCALEDIR)
   setlocale(LC_ALL, "");